Muse Keys
There are two Muse Keys hidden in the game, which are used to unlock a door in the Rings of Pandora. One is hidden in the underwater passage, after you've opened Poseidon's door; there will be a breakable wall that you can go through to reach an area with several enemies, breakable pots, and a chest containing the first key. The second key is found while going through the challenge of Poseidon, at the top of the ledges you must climb; simply go right when you've reached the top to find a chest with the second key. Once you have both keys, you can enter the room to obtain two Red Soul chests, and three fountains that will increase your maximum health, magic, and will give you five bars of Red Souls, respectively.

Unlockables
Making of GoW - Unlocked from beginning
Trailers(Teaser and Magic Teaser) - Unlocked from beginning
Credits - Beat game once (any difficulty)
Deleted Levels - Beat game once (any difficulty)
Heroic Possibilities - Beat game once (any difficulty)
Visions of Ancient Greece - Beat game once (any difficulty)
Monsters of Myth - Beat game once (any difficulty)
Birth of the - Beast Beat game once (any difficulty)
God Mode - Beat game once (any difficulty)
Character Graveyard - Beat game once (any difficulty)
Challenge of the Gods - Beat game once (any difficulty)
In-Game Movies - Beat game once (any difficulty)
A Secret Revealed - Beat the game on Spartan Mode
The Fate Of The Titan - Beat the game on Spartan Mode
Secret Message 1 - Beat the game on God Mode
Alteranate costumes for Krotos - Beat the Challenge Of The Gods

Secret 2 EU Version
In the European version of God of War you do not receive a telephone number scrambled up like in he American version. Instead you receive a secret cutscene which is ensued after destroying the Ares & Minotaur statues.